In driving a screw, it is often that the operator needs to know the tightness of the screw embedded into an object so as to determine whether a proper operation is achieved. If the screw is engaged too tight, it will cause it to be destroyed. If the screw is engaged too loose, it is possible that the screw is released from the object. Thereby in many applications, the hand tool is added with strain gauges for measuring the values of the twisting force applied thereto. Current hand tools are arranged with a strain gauge to measure the twisting force applied to the hand tool. In the prior art the sensitivity of the strain gauge is not so sensitive so that derived stresses are not precise and thus users cannot apply proper force according to the values. As a result, it is possible that the hand tool is destroyed or the screw means cannot be well fixed.
To improve above mentioned defect, in U.S. Pat. No. 3,970,155, a spanner with strain gauges is disclosed, where two strain gauges are installed at the driving portion and the driving head. The strain gauges are connected to a calculator and a display in the handle portion of the hand tool. However this design cannot precisely calculate the twisting value and thus the operator cannot get precise values.